Analysis of Generalized Multi-Protocol Label Switching (GMPLS)-based Recovery Mechanisms (including Protection and Restoration)
draft-ietf-ccamp-gmpls-recovery-analysis-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4428.
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|---|---|---|---|
| Authors | Dimitri Papadimitriou , Eric Mannie | ||
| Last updated | 2020-01-21 (Latest revision 2005-03-30) | ||
| Replaces | draft-papadimitriou-ccamp-gmpls-recovery-analysis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4428 (Informational) | |
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| Responsible AD | Alex D. Zinin | ||
| Send notices to | kireeti@juniper.net, adrian@olddog.co.uk |
draft-ietf-ccamp-gmpls-recovery-analysis-05
Network Working Group CCAMP GMPLS P&R Design Team
Internet Draft
Category: Informational Dimitri Papadimitriou (Editor)
Expiration Date: October 2005 Eric Mannie (Editor)
April 2005
Analysis of Generalized Multi-Protocol Label Switching (GMPLS)-based
Recovery Mechanisms (including Protection and Restoration)
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document provides an analysis grid to evaluate, compare and
contrast the Generalized Multi-Protocol Label Switching (GMPLS)
protocol suite capabilities with respect to the recovery mechanisms
currently proposed at the IETF CCAMP Working Group. A detailed
analysis of each of the recovery phases is provided using the
terminology defined in a companion document. This document focuses
on transport plane survivability and recovery issues and not on
control plane resilience and related aspects.
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Table of Content
Status of this Memo .............................................. 1
Abstract ......................................................... 1
Table of Content ................................................. 2
1. Contributors .................................................. 3
2. Conventions used in this Document ............................. 4
3. Introduction .................................................. 4
4. Fault Management .............................................. 5
4.1 Failure Detection ............................................ 5
4.2 Failure Localization and Isolation ........................... 7
4.3 Failure Notification ......................................... 8
4.4 Failure Correlation ......................................... 10
5. Recovery Mechanisms .......................................... 10
5.1 Transport vs. Control Plane Responsibilities ................ 10
5.2 Technology In/dependent Mechanisms .......................... 11
5.2.1 OTN Recovery .............................................. 11
5.2.2 Pre-OTN Recovery .......................................... 11
5.2.3 SONET/SDH Recovery ........................................ 12
5.3 Specific Aspects of Control Plane-based Recovery Mechanisms . 12
5.3.1 In-band vs. Out-of-band Signaling ......................... 12
5.3.2 Uni- vs. Bi-directional Failures .......................... 13
5.3.3 Partial vs. Full Span Recovery ............................ 15
5.3.4 Difference between LSP, LSP Segment and Span Recovery ..... 16
5.4 Difference between Recovery Type and Scheme ................. 16
5.5 LSP Recovery Mechanisms ..................................... 18
5.5.1 Classification ............................................ 18
5.5.2 LSP Restoration ........................................... 20
5.5.3 Pre-planned LSP Restoration ............................... 21
5.5.4 LSP Segment Restoration ................................... 22
6. Reversion .................................................... 23
6.1 Wait-To-Restore (WTR) ....................................... 23
6.2 Revertive Mode Operation .................................... 23
6.3 Orphans ..................................................... 24
7. Hierarchies .................................................. 24
7.1 Horizontal Hierarchy (Partitions) ........................... 25
7.2 Vertical Hierarchy (Layers) ................................. 25
7.2.1 Recovery Granularity ...................................... 27
7.3 Escalation Strategies ....................................... 27
7.4 Disjointness ................................................ 28
7.4.1 SRLG Disjointness ......................................... 28
8. Recovery Mechanisms Analysis ................................. 29
8.1 Fast Convergence (Detection/Correlation and Hold-off Time) .. 30
8.2 Efficiency (Recovery Switching Time) ........................ 30
8.3 Robustness .................................................. 31
8.4 Resource Optimization ....................................... 31
8.4.1 Recovery Resource Sharing ................................. 32
8.4.2 Recovery Resource Sharing and SRLG Recovery ............... 34
8.4.3 Recovery Resource Sharing, SRLG Disjointness and Admission. 35
9. Summary and Conclusions ...................................... 36
10. Security Considerations ..................................... 38
11. IANA Considerations ......................................... 38
12. Acknowledgments ............................................. 38
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13. References .................................................. 39
13.1 Normative References ....................................... 39
13.2 Informative References ..................................... 40
14. Editor's Address ............................................ 41
Intellectual Property Statement ................................. 42
Disclaimer of Validity .......................................... 42
Copyright Statement ............................................. 42
1. Contributors
This document is the result of the CCAMP Working Group Protection
and Restoration design team joint effort. Besides the editors, the
following are the authors that contributed to the present memo:
Deborah Brungard (AT&T)
200 S. Laurel Ave.
Middletown, NJ 07748, USA
EMail: dbrungard@att.com
Sudheer Dharanikota
EMail: sudheer@ieee.org
Jonathan P. Lang (Sonos)
506 Chapala Street
Santa Barbara, CA 93101, USA
EMail: jplang@ieee.org
Guangzhi Li (AT&T)
180 Park Avenue,
Florham Park, NJ 07932, USA
EMail: gli@research.att.com
Eric Mannie
EMail: eric_mannie@hotmail.com
Dimitri Papadimitriou (Alcatel)
Francis Wellesplein, 1
B-2018 Antwerpen, Belgium
EMail: dimitri.papadimitriou@alcatel.be
Bala Rajagopalan (Intel Broadband Wireless Division)
2111 NE 25th Ave.
Hillsboro, OR 97124, USA
EMail: bala.rajagopalan@intel.com
Yakov Rekhter (Juniper)
1194 N. Mathilda Avenue
Sunnyvale, CA 94089, USA
EMail: yakov@juniper.net
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2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
Any other recovery-related terminology used in this document
conforms to the one defined in [TERM]. The reader is also assumed to
be familiar with the terminology developed in [RFC3945], [RFC3471],
[RFC3473], [GMPLS-RTG] and [LMP].
3. Introduction
This document provides an analysis grid to evaluate, compare and
contrast the Generalized MPLS (GMPLS) protocol suite capabilities
with respect to the recovery mechanisms proposed at the IETF CCAMP
Working Group. The focus is on transport plane survivability and
recovery issues and not on control plane resilience related aspects.
Although the recovery mechanisms described in this document impose
different requirements on GMPLS-based recovery protocols, the
protocol(s) specifications will not be covered in this document.
Though the concepts discussed are technology independent, this
document implicitly focuses on SONET [T1.105]/SDH [G.707], Optical
Transport Networks (OTN) [G.709] and pre-OTN technologies except
when specific details need to be considered (for instance, in the
case of failure detection).
A detailed analysis is provided for each of the recovery phases as
identified in [TERM]. These phases define the sequence of generic
operations that need to be performed when a LSP/Span failure (or any
other event generating such failures) occurs:
- Phase 1: Failure detection
- Phase 2: Failure localization and isolation
- Phase 3: Failure notification
- Phase 4: Recovery (Protection/Restoration)
- Phase 5: Reversion (normalization)
Failure detection, localization and notification phases together are
referred to as fault management. Within a recovery domain, the
entities involved during the recovery operations are defined in
[TERM]; these entities include ingress, egress and intermediate
nodes. The term "recovery mechanism" is used to cover both
protection and restoration mechanisms. Specific terms such as
protection and restoration are only used when differentiation is
required. Likewise the term "failure" is used to represent both
signal failure and signal degradation.
In addition, a clear distinction is made between partitioning
(horizontal hierarchy) and layering (vertical hierarchy) when
analyzing the different hierarchical recovery mechanisms including
disjointness related issues. The dimensions from which each of the
recovery mechanisms detailed in this document can be analyzed are
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introduced to assess the current GMPLS protocol capabilities and the
potential need for further extensions. This document concludes by
detailing the applicability of the current GMPLS protocol building
blocks for recovery purposes.
4. Fault Management
4.1 Failure Detection
Transport failure detection is the only phase that can not be
achieved by the control plane alone since the latter needs a hook to
the transport plane to collect the related information. It has to be
emphasized that even if failure events themselves are detected by
the transport plane, the latter, upon a failure condition, must
trigger the control plane for subsequent actions through the use of
GMPLS signalling capabilities (see [RFC3471] and [RFC3473]) or Link
Management Protocol capabilities (see [LMP], Section 6).
Therefore, by definition, transport failure detection is transport
technology dependent (and so exceptionally, we keep here the
"transport plane" terminology). In transport fault management,
distinction is made between a defect and a failure. Here, the
discussion addresses failure detection (persistent fault cause). In
the technology-dependent descriptions, a more precise specification
will be provided.
As an example, SONET/SDH (see [G.707], [G.783] and [G.806]) provides
supervision capabilities covering:
- Continuity: monitors the integrity of the continuity of a trail
(i.e. section or path). This operation is performed by monitoring
the presence/absence of the signal. Examples are Loss of Signal
(LOS) detection for the physical layer, Unequipped (UNEQ) Signal
detection for the path layer, Server Signal Fail Detection (e.g.
AIS) at the client layer.
- Connectivity: monitors the integrity of the routing of the signal
between end-points. Connectivity monitoring is needed if
the layer provides flexible connectivity, either automatically
(e.g. cross-connects) or manually (e.g. fiber distribution frame).
An example is the Trail (i.e. section or path) Trace Identifier
used at the different layers and the corresponding Trail Trace
Identifier Mismatch detection.
- Alignment: checks that the client and server layer frame start can
be correctly recovered from the detection of loss of alignment.
The specific processes depend on the signal/frame structure and
may include: (multi-)frame alignment, pointer processing and
alignment of several independent frames to a common frame start in
case of inverse multiplexing. Loss of alignment is a generic term.
Examples are loss of frame, loss of multi-frame, or loss of
pointer.
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- Payload type: checks that compatible adaptation functions are used
at the source and the destination. This is normally done by adding
a payload type identifier (referred to as the "signal label") at
the source adaptation function and comparing it with the expected
identifier at the destination. For instance, the payload type
identifier and the corresponding mismatch detection.
- Signal Quality: monitors the performance of a signal. For
instance, if the performance falls below a certain threshold a
defect - excessive errors (EXC) or degraded signal (DEG) - is
detected.
The most important point is that the supervision processes and the
corresponding failure detection (used to initiate the recovery
phase(s)) result in either:
- Signal Degrade (SD): A signal indicating that the associated data
has degraded in the sense that a degraded defect condition is
active (for instance, a dDEG declared when the Bit Error Rate
exceeds a preset threshold).
- Signal Fail (SF): A signal indicating that the associated data has
failed in the sense that a signal interrupting near-end defect
condition is active (as opposed to the degraded defect).
In Optical Transport Networks (OTN) equivalent supervision
capabilities are provided at the optical/digital section layers
(i.e. Optical Transmission Section (OTS), Optical Multiplex Section
(OMS) and Optical channel Transport Unit (OTU)) and at the optical/
digital path layers (i.e. Optical Channel (OCh) and Optical channel
Data Unit (ODU)). Interested readers are referred to the ITU-T
Recommendations [G.798] and [G.709] for more details.
The above are examples that illustrate cases where the failure
detection, and reporting entities (see [TERM]) are co-located. The
following example illustrates the scenario where the failure
detecting and reporting entities (see [TERM]) are not co-located.
In pre-OTN networks, a failure may be masked by intermediate O-E-O
based Optical Line System (OLS), preventing a Photonic Cross-Connect
(PXC) from detecting upstream failures. In such cases, failure
detection may be assisted by an out-of-band communication channel
and failure condition reported to the PXC control plane. This can be
provided by using [LMP-WDM] extensions that delivers IP message-
based communication between the PXC and the OLS control plane. Also,
since PXCs are independent of the framing format, failure conditions
can only be triggered either by detecting the absence of the optical
signal or by measuring its quality. These mechanisms are generally
less reliable than electrical (digital) ones. Both types of
detection mechanisms are outside the scope of this document. If the
intermediate OLS supports electrical (digital) mechanisms, using the
LMP communication channel, these failure conditions are reported to
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the PXC and subsequent recovery actions performed as described in
Section 5. As such from the control plane viewpoint, this mechanism
turn the OLS-PXC composed system into a single logical entity
allowing the consideration of the same failure management mechanisms
for such entity as for any other O-E-O capable device.
More generally, the following are typical failure conditions in
SONET/SDH and pre-OTN networks:
- Loss of Light (LOL)/Loss of Signal (LOS): Signal Failure (SF)
condition where the optical signal is not detected any longer on
the receiver of a given interface.
- Signal Degrade (SD): detection of the signal degradation over
a specific period of time.
- For SONET/SDH payloads, all of the above-mentioned supervision
capabilities can be used, resulting in SD or SF condition.
In summary, the following cases apply when considering the
communication between the detecting and reporting entities:
- Co-located detecting and reporting entities: both the detecting
and reporting entities are on the same node (e.g., SONET/SDH
equipment, Opaque cross-connects, and, with some limitations,
Transparent cross-connects, etc.)
- Non co-located detecting and reporting entities:
o with in-band communication between entities: entities are
physically separated but the transport plane provides in-band
communication between them (e.g., Server Signal Failures (Alarm
Indication Signal (AIS)), etc.)
o with out-of-band communication between entities: entities are
physically separated but an out-of-band communication channel is
provided between them (e.g., using [LMP]).
4.2 Failure Localization and Isolation
Failure localization provides to the deciding entity information
about the location (and so the identity) of the transport plane
entity that detects the LSP(s)/span(s) failure. The deciding entity
can then make an accurate decision to achieve finer grained recovery
switching action(s). Note that this information can also be included
as part of the failure notification (see Section 4.3).
In some cases, this accurate failure localization information may be
less urgent to determine if it requires performing more time
consuming failure isolation (see also Section 4.4). This is
particularly the case when edge-to-edge LSP recovery (edge referring
to a sub-network end-node for instance) is performed based on a
simple failure notification (including the identification of the
working LSPs under failure condition). In this case, a more accurate
localization and isolation can be performed after recovery of these
LSPs.
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Failure localization should be triggered immediately after the fault
detection phase. This operation can be performed at the transport
plane and/or, if unavailable (via the transport plane), the control
plane level where dedicated signaling messages can be used. When
performed at the control plane level, a protocol such as LMP (see
[LMP], Section 6) can be used for failure localization purposes.
4.3 Failure Notification
Failure notification is used 1) to inform intermediate nodes that an
LSP/span failure has occurred and has been detected 2) to inform the
deciding entities (which can correspond to any intermediate or end-
point of the failed LSP/span) that the corresponding service is not
available. In general, these deciding entities will be the ones
taking the appropriate recovery decision. When co-located with the
recovering entity, these entities will also perform the
corresponding recovery action(s).
Failure notification can be either provided by the transport or by
the control plane. As an example, let us first briefly describe the
failure notification mechanism defined at the SONET/SDH transport
plane level (also referred to as maintenance signal supervision):
- AIS (Alarm Indication Signal) occurs as a result of a failure
condition such as Loss of Signal and is used to notify downstream
nodes (of the appropriate layer processing) that a failure has
occurred. AIS performs two functions 1) inform the intermediate
nodes (with the appropriate layer monitoring capability) that a
failure has been detected 2) notify the connection end-point that
the service is no longer available.
For a distributed control plane supporting one (or more) failure
notification mechanism(s), regardless of the mechanism's actual
implementation, the same capabilities are needed with more (or less)
information provided about the LSPs/spans under failure condition,
their detailed status, etc.
The most important difference between these mechanisms is related to
the fact that transport plane notifications (as defined today) would
directly initiate either a certain type of protection switching
(such as those described in [TERM]) via the transport plane or
restoration actions via the management plane.
On the other hand, using a failure notification mechanism through
the control plane would provide the possibility to trigger either a
protection or a restoration action via the control plane. This has
the advantage that a control plane recovery responsible entity does
not necessarily have to be co-located with a transport
maintenance/recovery domain. A control plane recovery domain can be
defined at entities not supporting a transport plane recovery.
Moreover, as specified in [RFC3473], notification message exchanges
through a GMPLS control plane may not follow the same path as the
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LSP/spans for which these messages carry the status. In turn, this
ensures a fast, reliable (through acknowledgement and the use of
either a dedicated control plane network or disjoint control
channels) and efficient (through the aggregation of several LSP/span
status within the same message) failure notification mechanism.
The other important properties to be met by the failure notification
mechanism are mainly the following:
- Notification messages must provide enough information such that
the most efficient subsequent recovery action will be taken (in
most of the recovery types and schemes this action is even
deterministic) at the recovering entities. Remember here that
these entities can be either intermediate or end-points through
which normal traffic flows. Based on local policy, intermediate
nodes may not use this information for subsequent recovery actions
(see for instance the APS protocol phases as described in [TERM]).
In addition, since fast notification is a mechanism running in
collaboration with the existing GMPLS signalling (see [RFC3473])
that also allows intermediate nodes to stay informed about the
status of the working LSP/spans under failure condition.
The trade-off here is to define what information the LSP/span end-
points (more precisely, the deciding entity) needs in order for
the recovering entity to take the best recovery action: if not
enough information is provided, the decision can not be optimal
(note that in this eventuality, the important issue is to quantify
the level of sub-optimality), if too much information is provided
the control plane may be overloaded with unnecessary information
and the aggregation/correlation of this notification information
will be more complex and time consuming to achieve. Note that a
more detailed quantification of the amount of information to be
exchanged and processed is strongly dependent on the failure
notification protocol.
- If the failure localization and isolation is not performed by one
of the LSP/span end-points or some intermediate points, they
should receive enough information from the notification message in
order to locate the failure otherwise they would need to (re-)
initiate a failure localization and isolation action.
- Avoiding so-called notification storms implies that 1) the failure
detection output is correlated (i.e. alarm correlation) and
aggregated at the node detecting the failure(s) 2) the failure
notifications are directed to a restricted set of destinations (in
general the end-points) and that 3) failure notification
suppression (i.e. alarm suppression) is provided in order to limit
flooding in case of multiple and/or correlated failures appearing
at several locations in the network.
- Alarm correlation and aggregation (at the failure detecting
node) implies a consistent decision based on the conditions for
which a trade-off between fast convergence (at detecting node) and
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fast notification (implying that correlation and aggregation
occurs at receiving end-points) can be found.
4.4 Failure Correlation
A single failure event (such as a span failure) can result into
reporting multiple failures (such as individual LSP failures)
conditions. These can be grouped (i.e. correlated) to reduce the
number of failure conditions communicated on the reporting channel,
for both in-band and out-of-band failure reporting.
In such a scenario, it can be important to wait for a certain period
of time, typically called failure correlation time, and gather all
the failures to report them as a group of failures (or simply group
failure). For instance, this approach can be provided using LMP-WDM
for pre-OTN networks (see [LMP-WDM]) or when using Signal Failure/
Degrade Group in the SONET/SDH context.
Note that a default average time interval during which failure
correlation operation can be performed is difficult to provide since
it is strongly dependent on the underlying network topology.
Therefore, it can be advisable to provide a per-node configurable
failure correlation time. The detailed selection criteria for this
time interval are outside of the scope of this document.
When failure correlation is not provided, multiple failure
notification messages may be sent out in response to a single
failure (for instance, a fiber cut), each one containing a set of
information on the failed working resources (for instance, the
individual lambda LSP flowing through this fiber). This allows for a
more prompt response but can potentially overload the control plane
due to a large amount of failure notifications.
5. Recovery Mechanisms
5.1 Transport vs. Control Plane Responsibilities
For both protection and restoration, and when applicable, recovery
resources are provisioned using GMPLS signalling capabilities. Thus,
these are control plane-driven actions (topological and resource-
constrained) that are always performed in this context.
The following tables give an overview of the responsibilities taken
by the control plane in case of LSP/span recovery:
1. LSP/span Protection
- Phase 1: Failure detection Transport plane
- Phase 2: Failure localization/isolation Transport/Control plane
- Phase 3: Failure notification Transport/Control plane
- Phase 4: Protection switching Transport/Control plane
- Phase 5: Reversion (normalization) Transport/Control plane
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Note: in the context of LSP/span protection, control plane actions
can be performed either for operational purposes and/or
synchronization purposes (vertical synchronization between transport
and control plane) and/or notification purposes (horizontal
synchronization between end-nodes at control plane level). This
suggests the selection of the responsible plane (in particular for
protection switching) during the provisioning phase of the
protected/protection LSP.
2. LSP/span Restoration
- Phase 1: Failure detection Transport plane
- Phase 2: Failure localization/isolation Transport/Control plane
- Phase 3: Failure notification Control plane
- Phase 4: Recovery switching Control plane
- Phase 5: Reversion (normalization) Control plane
Therefore, this document primarily focuses on provisioning of LSP
recovery resources, failure notification mechanisms, recovery
switching, and reversion operations. Moreover some additional
considerations can be dedicated to the mechanisms associated to the
failure localization/isolation phase.
5.2 Technology in/dependent mechanisms
The present recovery mechanisms analysis applies in fact to any
circuit oriented data plane technology with discrete bandwidth
increments (like SONET/SDH, G.709 OTN, etc.) being controlled by a
GMPLS-based distributed control plane.
The following sub-sections are not intended to favor one technology
versus another. They just list pro and cons for each of them in
order to determine the mechanisms that GMPLS-based recovery must
deliver to overcome their cons and take benefits of their pros in
their respective applicability context.
5.2.1 OTN Recovery
OTN recovery specifics are left for further considerations.
5.2.2 Pre-OTN Recovery
Pre-OTN recovery specifics (also referred to as "lambda switching")
present mainly the following advantages:
- benefits from a simpler architecture making it more suitable for
mesh-based recovery types and schemes (on a per channel basis).
- when providing suppression of intermediate node transponders (vs.
use of non-standard masking of upstream failures) e.g. use of
squelching, implies that failures (such as LoL) will propagate to
edge nodes giving the possibility to initiate recovery actions
driven by upper layers.
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The main disadvantage comes from the lack of interworking due to the
large amount of failure management (in particular failure
notification protocols) and recovery mechanisms currently available.
Note also, that for all-optical networks, combination of recovery
with optical physical impairments is left for a future release of
this document since corresponding detection technologies are under
specification.
5.2.3 SONET/SDH Recovery
Some of the advantages of SONET [T1.105]/SDH [G.707] and more
generically any TDM transport plane recovery are that they provide:
- Protection types operating at the data plane level are
standardized (see [G.841]) and can operate across protected
domains and interwork (see [G.842]).
- Failure detection, notification and path/section Automatic
Protection Switching (APS) mechanisms.
- Greater control over the granularity of the TDM LSPs/links that
can be recovered with respect to coarser optical channel (or whole
fiber content) recovery switching
Some of the limitations of the SONET/SDH recovery are:
- Limited topological scope: Inherently the use of ring topologies,
typically, dedicated Sub-Network Connection Protection (SNCP) or
shared protection rings, has reduced flexibility and resource
efficiency with respect to the (somewhat more complex) meshed
recovery.
- Inefficient use of spare capacity: SONET/SDH protection is largely
applied to ring topologies, where spare capacity often remains
idle, making the efficiency of bandwidth usage a real issue.
- Support of meshed recovery requires intensive network management
development and the functionality is limited by both the network
elements and the capabilities of the element management systems
(justifying thus the development of GMPLS-based distributed
recovery mechanisms.)
5.3 Specific Aspects of Control Plane-based Recovery Mechanisms
5.3.1 In-band vs Out-of-band Signalling
The nodes communicate through the use of IP terminating control
channels defining the control plane (transport) topology. In this
context, two classes of transport mechanisms can be considered here
i.e. in-fiber or out-of-fiber (through a dedicated physically
diverse control network referred to as the Data Communication
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Network or DCN). The potential impact of the usage of an in-fiber
(signalling) transport mechanism is briefly considered here.
In-fiber transport mechanism can be further subdivided into in-band
and out-of-band. As such, the distinction between in-fiber in-band
and in-fiber out-of-band signalling reduces to the consideration of
a logically versus physically embedded control plane topology with
respect to the transport plane topology. In the scope of this
document, it is assumed that at least one IP control channel between
each pair of adjacent nodes is continuously available to enable the
exchange of recovery-related information and messages. Thus, in
either case (i.e. in-band or out-of-band) at least one logical or
physical control channel between each pair of nodes is always
expected to be available.
Therefore, the key issue when using in-fiber signalling is whether
one can assume independence between the fault-tolerance capabilities
of control plane and the failures affecting the transport plane
(including the nodes). Note also that existing specifications like
the OTN provide a limited form of independence for in-fiber
signaling by dedicating a separate optical supervisory channel (OSC,
see [G.709] and [G.874]) to transport the overhead and other control
traffic. For OTNs, failure of the OSC does not result in failing the
optical channels. Similarly, loss of the control channel must not
result in failing the data channels (transport plane).
5.3.2 Uni- versus Bi-directional Failures
The failure detection, correlation and notification mechanisms
(described in Section 4) can be triggered when either a
unidirectional or a bi-directional LSP/Span failure occurs (or a
combination of both). As illustrated in Figure 1 and 2, two
alternatives can be considered here:
1. Uni-directional failure detection: the failure is detected on the
receiver side i.e. it is only is detected by the downstream node
to the failure (or by the upstream node depending on the failure
propagation direction, respectively).
2. Bi-directional failure detection: the failure is detected on the
receiver side of both downstream node AND upstream node to the
failure.
Notice that after the failure detection time, if only control plane
based failure management is provided, the peering node is unaware of
the failure detection status of its neighbor.
------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |---------| |----...----| |
------- ------- ------- -------
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t0 >>>>>>> F
t1 x <---------------x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
------- ------- ------- -------
| | | |Tx Rx| | | |
| NodeA |----...----| NodeB |xxxxxxxxx| NodeC |----...----| NodeD |
| |----...----| |xxxxxxxxx| |----...----| |
------- ------- ------- -------
t0 F <<<<<<< >>>>>>> F
t1 x <-------------> x
Notification
t2 <--------...--------x x--------...-------->
Up Notification Down Notification
After failure detection, the following failure management operations
can be subsequently considered:
- Each detecting entity sends a notification message to the
corresponding transmitting entity. For instance, in Fig. 1 (Fig.
2), node C sends a notification message to node B (while node B
sends a notification message to node C). To ensure reliable
failure notification, a dedicated acknowledgment message can be
returned back to the sender node.
- Next, within a certain (and pre-determined) time window, nodes
impacted by the failure occurrences may perform their correlation.
In case of unidirectional failure, node B only receives the
notification message from node C and thus the time for this
operation is negligible. In case of bi-directional failure, node B
(and node C) has to correlate the received notification message
from node C (node B, respectively) with the corresponding locally
detected information.
- After some (pre-determined) period of time, referred to as the
hold-off time, after which the local recovery actions (see Section
5.3.4) were not successful, the following occurs. In case of
unidirectional failure and depending on the directionality of the
LSP, node B should send an upstream notification message (see
[RFC3473]) to the ingress node A and node C may send a downstream
notification message (see [RFC3473]) to the egress node D.
However, in such a case only node A referred to as the "master"
(node D being then referred to as the "slave" per [TERM]), would
initiate an edge to edge recovery action. Note that the other LSP
end-node (i.e. node D in this case) may be optionally notified
using a downstream notification message (see [RFC3473]).
In case of bi-directional failure, node B should send an upstream
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notification message (see [RFC3473]) to the ingress node A and
node C may send a downstream notification message (see [RFC3473])
to the egress node D. However, due to the dependence on the LSP
directionality, only ingress node A would initiate an edge to edge
recovery action. Note that the other LSP end-node (i.e. node D in
this case) should also be notified of this event using a
downstream notification message (see [RFC3473]). For instance, if
an LSP directed from D to A is under failure condition, only the
notification message sent from node C to D would initiate a
recovery action and, in this case, per [TERM], the deciding and
recovering node D is referred to as the "master" while node A is
referred to as the "slave" (i.e. recovering only entity).
Note: The determination of the master and the slave may be based
either on configured information or dedicated protocol capability.
In the above scenarios, the path followed by the upstream and
downstream notification messages does not have to be the same as the
one followed by the failed LSP (see [RFC3473] for more details on
the notification message exchange). The important point, concerning
this mechanism, is that either the detecting/reporting entity (i.e.
the nodes B and C) is also the deciding/recovery entity or the
detecting/reporting entity is simply an intermediate node in the
subsequent recovery process. One refers to local recovery in the
former case and to edge-to-edge recovery in the latter one (see also
Section 5.3.4).
5.3.3 Partial versus Full Span Recovery
When a given span carries more than one LSPs or LSP segments, an
additional aspect must be considered. In case of span failure, the
LSPs it carries can be either individually recovered or recovered as
a group (aka bulk LSP recovery) or independent sub-groups. The
selection of this mechanism would be triggered independently of the
failure notification granularity when correlation time windows are
used and simultaneous recovery of several LSPs can be performed
using a single request. Moreover, criteria by which such sub-groups
can be formed are outside of the scope of this document.
Additional complexity arises in the case of (sub-)group LSP
recovery. Between a given pair of nodes, the LSPs that a given (sub-
)group contains may have been created from different source nodes
(i.e. initiator) and directed toward different destinations nodes.
Consequently the failure notification messages sub-sequent to a bi-
directional span failure affecting several LSPs (or the whole group
of LSPs it carries) are not necessarily directed toward the same
initiator nodes. In particular these messages may be directed to
both the upstream and downstream nodes to the failure. Therefore,
such span failure may trigger recovery actions to be performed from
both sides (i.e. both from the upstream and the downstream node to
the failure). In order to facilitate the definition of the
corresponding recovery mechanisms (and their sequence), one assumes
here as well, that per [TERM] the deciding (and recovering) entity,
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referred to as the "master" is the only initiator of the recovery of
the whole LSP (sub-)group.
5.3.4 Difference between LSP, LSP Segment and Span Recovery
The recovery definitions given in [TERM] are quite generic and apply
for link (or local span) and LSP recovery. The major difference
between LSP, LSP Segment and span recovery is related to the number
of intermediate nodes that the signalling messages have to travel.
Since nodes are not necessarily adjacent in case of LSP (or LSP
Segment) recovery, signalling message exchanges from the reporting
to the deciding/recovery entity may have to cross several
intermediate nodes. In particular, this applies for the notification
messages due to the number of hops separating the location of a
failure occurrence from its destination. This results in an
additional propagation and forwarding delay. Note that the former
delay may in certain circumstances be non-negligible; e.g. in case
of copper out-of-band network, approximately 1 ms per 200km.
Moreover, the recovery mechanisms applicable to end-to-end LSPs and
to the segments that may compose an end-to-end LSP (i.e. edge-to-
edge recovery) can be exactly the same. However, one expects in the
latter case, that the destination of the failure notification
message will be the ingress/egress of each of these segments.
Therefore, using the mechanisms described in Section 5.3.2, failure
notification messages can be first exchanged between terminating
points of the LSP segment and after expiration of the hold-off time
be directed toward terminating points of the end-to-end LSP.
Note: Several studies provide quantitative analysis of the relative
performance of LSP/span recovery techniques. [WANG] for instance,
provides an analysis grid for these techniques showing that dynamic
LSP restoration (see Section 5.5.2) performs well under medium
network loads but suffers performance degradations at higher loads
due to greater contention for recovery resources. LSP restoration
upon span failure, as defined in [WANG], degrades at higher loads
because paths around failed links tend to increase the hop count of
the affected LSPs and thus consume additional network resources.
Also, performance of LSP restoration can be enhanced by a failed
working LSP's source node initiating a new recovery attempt if an
initial attempt fails. A single retry attempt is sufficient to
produce large increases in the restoration success rate and ability
to initiate successful LSP restoration attempts, especially at high
loads, while not adding significantly to the long-term average
recovery time. Allowing additional attempts produces only small
additional gains in performance. This suggests using additional
(intermediate) crankback signalling when using dynamic LSP
restoration (described in Section 5.5.2 - case 2). Details on
crankback signalling are outside the scope of the present document.
5.4 Difference between Recovery Type and Scheme
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[TERM] defines the basic LSP/span recovery types. This section
describes the recovery schemes that can be built using these
recovery types. In brief, a recovery scheme is defined as the
combination of several ingress-egress node pairs supporting a given
recovery type (from the set of the recovery types they allow).
Several examples are provided here to illustrate the difference
between recovery types such as 1:1 or M:N and recovery schemes such
as (1:1)^n or (M:N)^n referred to as shared-mesh recovery.
1. (1:1)^n with recovery resource sharing
The exponent, n, indicates the number of times a 1:1 recovery type
is applied between at most n different ingress-egress node pairs.
Here, at most n pairs of disjoint working and recovery LSPs/spans
share at most n times a common resource. Since the working LSPs/
spans are mutually disjoint, simultaneous requests for use of the
shared (common) resource will only occur in case of simultaneous
failures, which is less likely to happen.
For instance, in the common (1:1)^2 case, if the 2 recovery LSPs in
the group overlap the same common resource, then it can handle only
single failures; any multiple working LSP failures will cause at
least one working LSP to be denied automatic recovery. Consider for
instance the following topology with the working LSPs A-B-C and F-G-
H and their respective recovery LSPs A-D-E-C and F-D-E-H that share
a common D-E link resource.
A---------B---------C
\ /
\ /
D-------------E
/ \
/ \
F---------G---------H
2. (M:N)^n with recovery resource sharing
The (M:N)^n scheme is documented here for the sake of completeness
only (i.e. it is not mandated that GMPLS capabilities would support
this scheme). The exponent, n, indicates the number of times an M:N
recovery type is applied between at most n different ingress-egress
node pairs. So the interpretation follows from the previous case,
except that here disjointness applies to the N working LSPs/spans
and to the M recovery LSPs/spans while sharing at most n times M
common resources.
In both schemes, it results in a "group" of sum{n=1}^N N{n} working
LSPs and a pool of shared recovery resources, not all of which are
available to any given working LSP. In such conditions, defining a
metric that describes the amount of overlap among the recovery LSPs
would give some indication of the group's ability to handle
simultaneous failures of multiple LSPs.
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For instance, in the simple (1:1)^n case, if n recovery LSPs in a
(1:1)^n group overlap, then it can handle only single failures; any
simultaneous failure of multiple working LSPs will cause at least
one working LSP to be denied automatic recovery. But if one
considers for instance, a (2:2)^2 group in which there are two pairs
of overlapping recovery LSPs, then two LSPs (belonging to the same
pair) can be simultaneously recovered. The latter case can be
illustrated by the following topology with 2 pairs of working LSPs
A-B-C and F-G-H and their respective recovery LSPs A-D-E-C and F-D-
E-H that share two common D-E link resources.
A========B========C
\\ //
\\ //
D =========== E
// \\
// \\
F========G========H
Moreover, in all these schemes, (working) path disjointness can be
enforced by exchanging information related to working LSPs during
the recovery LSP signaling. Specific issues related to the
combination of shared (discrete) bandwidth and disjointness for
recovery schemes are described in Section 8.4.2.
5.5 LSP Recovery Mechanisms
5.5.1 Classification
The recovery time and ratio of LSPs/spans depend on proper recovery
LSP provisioning (meaning pre-provisioning when performed before
failure occurrence) and the level of overbooking of recovery
resources (i.e. over-provisioning). A proper balance of these two
operations will result in the desired LSP/span recovery time and
ratio when single or multiple failure(s) occur(s). Note also that
these operations are mostly performed during the network planning
phases.
The different options for LSP (pre-)provisioning and overbooking are
classified below to structure the analysis of the different recovery
mechanisms.
1. Pre-Provisioning
Proper recovery LSP pre-provisioning will help to alleviate the
failure of the working LSPs (due to the failure of the resources
that carry these LSPs). As an example, one may compute and establish
the recovery LSP either end-to-end or segment-per-segment, to
protect a working LSP from multiple failure events affecting
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link(s), node(s) and/or SRLG(s). The recovery LSP pre-provisioning
options can be classified (in the below figure) as follows:
(1) the recovery path can be either pre-computed or computed
on-demand.
(2) when the recovery path is pre-computed, it can be either pre-
signaled (implying recovery resource reservation) or signaled
on-demand.
(3) when the recovery resources are pre-signaled, they can be either
pre-selected or selected on-demand.
Recovery LSP provisioning phases:
(1) Path Computation --> On-demand
|
|
--> Pre-Computed
|
|
(2) Signalling --> On-demand
|
|
--> Pre-Signaled
|
|
(3) Resource Selection --> On-demand
|
|
--> Pre-Selected
Note that these different options lead to different LSP/span
recovery times. The following sections will consider the above-
mentioned pre-provisioning options when analyzing the different
recovery mechanisms.
2. Overbooking
There are many mechanisms available that allow the overbooking of
the recovery resources. This overbooking can be done per LSP (such
as the example mentioned above), per link (such as span protection)
or even per domain. In all these cases, the level of overbooking, as
shown in the below figure, can be classified as dedicated (such as
1+1 and 1:1), shared (such as 1:N and M:N) or unprotected (and thus
restorable if enough recovery resources are available).
Overbooking levels:
+----- Dedicated (for instance: 1+1, 1:1, etc.)
|
|
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+----- Shared (for instance: 1:N, M:N, etc.)
|
Level of |
Overbooking -----+----- Unprotected (for instance: 0:1, 0:N)
Also, when using shared recovery, one may support preemptible extra-
traffic; the recovery mechanism is then expected to allow preemption
of this low priority traffic in case of recovery resource contention
during recovery operations. The following sections will consider the
above-mentioned overbooking options when analyzing the different
recovery mechanisms.
5.5.2 LSP Restoration
The following times are defined to provide a quantitative estimation
about the time performance of the different LSP restoration
mechanisms (also referred to as LSP re-routing):
- Path Computation Time: Tc
- Path Selection Time: Ts
- End-to-end LSP Resource Reservation Time: Tr (a delta for resource
selection is also considered, the corresponding total time is then
referred to as Trs)
- End-to-end LSP Resource Activation Time: Ta (a delta for
resource selection is also considered, the corresponding total
time is then referred to as Tas)
The Path Selection Time (Ts) is considered when a pool of recovery
LSP paths between a given pair of source/destination end-points is
pre-computed and after a failure occurrence one of these paths is
selected for the recovery of the LSP under failure condition.
Note: failure management operations such as failure detection,
correlation and notification are considered (for a given failure
event) as equally time consuming for all the mechanisms described
here below:
1. With Route Pre-computation (or LSP re-provisioning)
An end-to-end restoration LSP is established after the failure(s)
occur(s) based on a pre-computed path. As such, one can define this
as an "LSP re-provisioning" mechanism. Here, one or more (disjoint)
paths for the restoration LSP are computed (and optionally pre-
selected) before a failure occurs.
No reservation or selection of resources is performed along the
restoration path before failure occurrence. As a result, there is no
guarantee that a restoration LSP is available when a failure occurs.
The expected total restoration time T is thus equal to Ts + Trs or
to Trs when a dedicated computation is performed for each working
LSP.
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2. Without Route Pre-computation (or Full LSP re-routing)
An end-to-end restoration LSP is dynamically established after the
failure(s) occur(s). Here, after failure occurrence, one or more
(disjoint) paths for the restoration LSP are dynamically computed
and one is selected. As such, one can define this as a complete "LSP
re-routing" mechanism.
No reservation or selection of resources is performed along the
restoration path before failure occurrence. As a result, there is no
guarantee that a restoration LSP is available when a failure occurs.
The expected total restoration time T is thus equal to Tc (+ Ts) +
Trs. Therefore, time performance between these two approaches
differs by the time required for route computation Tc (and its
potential selection time, Ts).
5.5.3 Pre-planned LSP Restoration
Pre-planned LSP restoration (also referred to as pre-planned LSP re-
routing) implies that the restoration LSP is pre-signaled. This in
turn implies the reservation of recovery resources along the
restoration path. Two cases can be defined based on whether the
recovery resources are pre-selected or not.
1. With resource reservation and without resource pre-selection
Before failure occurrence, an end-to-end restoration path is pre-
selected from a set of pre-computed (disjoint) paths. The
restoration LSP is signaled along this pre-selected path to reserve
resources at each node but these resources are not selected.
In this case, the resources reserved for each restoration LSP may be
dedicated or shared between multiple restoration LSPs whose working
LSPs are not expected to fail simultaneously. Local node policies
can be applied to define the degree to which these resources can be
shared across independent failures. Also, since a restoration scheme
is considered, resource sharing should not be limited to restoration
LSPs starting and ending at the same ingress and egress nodes.
Therefore, each node participating to this scheme is expected to
receive some feedback information on the sharing degree of the
recovery resource(s) that this scheme involves.
Upon failure detection/notification message reception, signaling is
initiated along the restoration path to select the resources, and to
perform the appropriate operation at each node crossed by the
restoration LSP (e.g. cross-connections). If lower priority LSPs
were established using the restoration resources, they must be
preempted when the restoration LSP is activated.
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The expected total restoration time T is thus equal to Tas (post-
failure activation) while operations performed before failure
occurrence takes Tc + Ts + Tr.
2. With both resource reservation and resource pre-selection
Before failure occurrence, an end-to-end restoration path is pre-
selected from a set of pre-computed (disjoint) paths. The
restoration LSP is signaled along this pre-selected path to reserve
AND select resources at each node but these resources are not
committed at the data plane level. Such that the selection of the
recovery resources is committed at the control plane level only, no
cross-connections are performed along the restoration path.
In this case, the resources reserved and selected for each
restoration LSP may be dedicated or even shared between multiple
restoration LSPs whose associated working LSPs are not expected to
fail simultaneously. Local node policies can be applied to define
the degree to which these resources can be shared across independent
failures. Also, since a restoration scheme is considered, resource
sharing should not be limited to restoration LSPs starting and
ending at the same ingress and egress nodes. Therefore, each node
participating to this scheme is expected to receive some feedback
information on the sharing degree of the recovery resource(s) that
this scheme involves.
Upon failure detection/notification message reception, signaling is
initiated along the restoration path to activate the reserved and
selected resources, and to perform the appropriate operation at each
node crossed by the restoration LSP (e.g. cross-connections). If
lower priority LSPs were established using the restoration
resources, they must be preempted when the restoration LSP is
activated.
The expected total restoration time T is thus equal to Ta (post-
failure activation) while operations performed before failure
occurrence takes Tc + Ts + Trs. Therefore, time performance between
these two approaches differs only by the time required for resource
selection during the activation of the recovery LSP (i.e. Tas - Ta).
5.5.4 LSP Segment Restoration
The above approaches can be applied on an edge-to-edge LSP basis
rather than end-to-end LSP basis (i.e. to reduce the global recovery
time) by allowing the recovery of the individual LSP segments
constituting the end-to-end LSP.
Also, by using the horizontal hierarchy approach described in
Section 7.1, an end-to-end LSP can be recovered by multiple recovery
mechanisms applied on an LSP segment basis (e.g. 1:1 edge-to-edge
LSP protection in a metro network and M:N edge-to-edge protection in
the core). These mechanisms are ideally independent and may even use
different failure localization and notification mechanisms.
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6. Reversion
Reversion (a.k.a. normalization) is defined as the mechanism
allowing switching of normal traffic from the recovery LSP/span to
the working LSP/span previously under failure condition. Use of
normalization is at the discretion of the recovery domain policy.
Normalization (also referred to as reversion) may impact the normal
traffic (a second hit) depending on the normalization mechanism
used.
If normalization is supported 1) the LSP/span must be returned to
the working LSP/span when the failure condition clears 2) the
capability to de-activate (turn-off) the use of reversion should be
provided. De-activation of reversion should not impact the normal
traffic regardless of whether currently using the working or
recovery LSP/span.
Note: during the failure, the reuse of any non-failed resources
(e.g. LSP and/or spans) belonging to the working LSP/span is under
the discretion of recovery domain policy.
6.1 Wait-To-Restore (WTR)
A specific mechanism (Wait-To-Restore) is used to prevent frequent
recovery switching operations due to an intermittent defect (e.g.
BER fluctuating around the SD threshold).
First, an LSP/span under failure condition must become fault-free,
e.g. a BER less than a certain recovery threshold. After the
recovered LSP/span (i.e. the previously working LSP/span) meets this
criterion, a fixed period of time shall elapse before normal traffic
uses the corresponding resources again. This duration called Wait-
To-Restore (WTR) period or timer is generally of the order of a few
minutes (for instance, 5 minutes) and should be capable of being
set. The WTR timer may be either a fixed period, or provide for
incrementally longer periods before retrying. An SF or SD condition
on the previously working LSP/span will override the WTR timer value
(i.e. the WTR cancels and the WTR timer will restart).
6.2 Revertive Mode Operation
In revertive mode of operation, when the recovery LSP/span is no
longer required, i.e. the failed working LSP/span is no longer in SD
or SF condition, a local Wait-to-Restore (WTR) state will be
activated before switching the normal traffic back to the recovered
working LSP/span.
During the reversion operation, since this state becomes the highest
in priority, signalling must maintain the normal traffic on the
recovery LSP/span from the previously failed working LSP/span.
Moreover, during this WTR state, any null traffic or extra traffic
(if applicable) request is rejected.
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However, deactivation (cancellation) of the wait-to-restore timer
may occur in case of higher priority request attempts. That is the
recovery LSP/span usage by the normal traffic may be preempted if a
higher priority request for this recovery LSP/span is attempted.
6.3 Orphans
When a reversion operation is requested normal traffic must be
switched from the recovery to the recovered working LSP/span. A
particular situation occurs when the previously working LSP/span
cannot be recovered such that normal traffic can not be switched
back. In such a case, the LSP/span under failure condition (also
referred to as "orphan") must be cleared i.e. removed from the pool
of resources allocated for normal traffic. Otherwise, potential de-
synchronization between the control and transport plane resource
usage can appear. Depending on the signalling protocol capabilities
and behavior different mechanisms are expected here.
Therefore any reserved or allocated resources for the LSP/span under
failure condition must be unreserved/de-allocated. Several ways can
be used for that purpose: either wait for the elapsing of the clear-
out time interval, or initiate a deletion from the ingress or the
egress node, or trigger the initiation of deletion from an entity
(such as an EMS or NMS) capable to react on the reception of an
appropriate notification message.
7. Hierarchies
Recovery mechanisms are being made available at multiple (if not
each) transport layers within so-called "IP/MPLS-over-optical"
networks. However, each layer has certain recovery features and one
needs to determine the exact impact of the interaction between the
recovery mechanisms provided by these layers.
Hierarchies are used to build scalable complex systems. Abstraction
is used as a mechanism to build large networks or as a technique for
enforcing technology, topological or administrative boundaries by
hiding the internal details. The same hierarchical concept can be
applied to control the network survivability. Network survivability
is the set of capabilities that allow a network to restore affected
traffic in the event of a failure. Network survivability is defined
further in [TERM]. In general, it is expected that the recovery
action is taken by the recoverable LSP/span closest to the failure
in order to avoid the multiplication of recovery actions. Moreover,
recovery hierarchies can be also bound to control plane logical
partitions (e.g. administrative or topological boundaries). Each of
them may apply different recovery mechanisms.
In brief, the commonly accepted ideas are generally that the lower
layers can provide coarse but faster recovery while the higher
layers can provide finer but slower recovery. Moreover, it is also
desirable to avoid similar layers with functional overlaps to
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optimize network resource utilization and processing overhead, since
repeating the same capabilities at each layer does not create any
added value for the network as a whole. In addition, even if a lower
layer recovery mechanism is enabled, doing so does not prevent the
additional provision of a recovery mechanism at the upper layer. The
inverse statement does not necessarily hold; that is, enabling an
upper layer recovery mechanism may prevent the use of a lower layer
recovery mechanism. In this context, this section intends to analyze
these hierarchical aspects including the physical (passive)
layer(s).
7.1 Horizontal Hierarchy (Partitioning)
A horizontal hierarchy is defined when partitioning a single-layer
network (and its control plane) into several recovery domains.
Within a domain, the recovery scope may extend over a link (or
span), LSP segment or even an end-to-end LSP. Moreover, an
administrative domain may consist of a single recovery domain or can
be partitioned into several smaller recovery domains. The operator
can partition the network into recovery domains based on physical
network topology, control plane capabilities or various traffic
engineering constraints.
An example often addressed in the literature is the metro-core-metro
application (sometimes extended to a metro-metro/core-core) within a
single transport layer (see Section 7.2). For such a case, an end-
to-end LSP is defined between the ingress and egress metro nodes,
while LSP segments may be defined within the metro or core sub-
networks. Each of these topological structures determines a so-
called "recovery domain" since each of the LSPs they carry can have
its own recovery type (or even scheme). The support of multiple
recovery types and schemes within a sub-network is referred to as a
multi-recovery capable domain or simply multi-recovery domain.
7.2 Vertical Hierarchy (Layers)
It is a very challenging task to combine in a coordinated manner the
different recovery capabilities available across the path (i.e.
switching capable) and section layers to ensure that certain network
survivability objectives are met for the different services
supported by the network.
As a first analysis step, one can draw the following guidelines for
a vertical coordination of the recovery mechanisms:
- The lower the layer the faster the notification and switching
- The higher the layer the finer the granularity of the recoverable
entity and therefore the granularity of the recovery resource
Moreover, in the context of this analysis, a vertical hierarchy
consists of multiple layered transport planes providing different:
- Discrete bandwidth granularities for non-packet LSPs such as OCh,
ODUk, STS_SPE/HOVC and VT_SPE/LOVC LSPs and continuous bandwidth
granularities for packet LSPs
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- Potential recovery capabilities with different temporal
granularities: ranging from milliseconds to tens of seconds
Note: based on the bandwidth granularity we can determine four
classes of vertical hierarchies (1) packet over packet (2) packet
over circuit (3) circuit over packet and (4) circuit over circuit.
Below we briefly expand on (4) only. (2) is covered in [RFC3386].
(1) is extensively covered by the MPLS Working Group, and (3) by the
PWE3 Working Group.
In SONET/SDH environments, one typically considers the VT_SPE/LOVC
and STS SPE/HOVC as independent layers, VT_SPE/LOVC LSP using the
underlying STS_SPE/HOVC LSPs as links, for instance. In OTN, the
ODUk path layers will lie on the OCh path layer i.e. the ODUk LSPs
using the underlying OCh LSPs as OTUk links. Note here that lower
layer LSPs may simply be provisioned and not necessarily dynamically
triggered or established (control driven approach). In this context,
an LSP at the path layer (i.e. established using GMPLS signalling),
for instance an optical channel LSP, appears at the OTUk layer as a
link, controlled by a link management protocol such as LMP.
The first key issue with multi-layer recovery is that achieving
individual or bulk LSP recovery will be as efficient as the
underlying link (local span) recovery. In such a case, the span can
be either protected or unprotected, but the LSP it carries must be
(at least locally) recoverable. Therefore, the span recovery process
can be either independent when protected (or restorable), or
triggered by the upper LSP recovery process. The former case
requires coordination to achieve subsequent LSP recovery. Therefore,
in order to achieve robustness and fast convergence, multi-layer
recovery requires a fine-tuned coordination mechanism.
Moreover, in the absence of adequate recovery mechanism coordination
(for instance, a pre-determined coordination when using a hold-off
timer), a failure notification may propagate from one layer to the
next one within a recovery hierarchy. This can cause "collisions"
and trigger simultaneous recovery actions that may lead to race
conditions and in turn, reduce the optimization of the resource
utilization and/or generate global instabilities in the network (see
[MANCHESTER]). Therefore, a consistent and efficient escalation
strategy is needed to coordinate recovery across several layers.
Therefore, one can expect that the definition of the recovery
mechanisms and protocol(s) is technology-independent such that they
can be consistently implemented at different layers; this would in
turn simplify their global coordination. Moreover, as mentioned in
[RFC3386], some looser form of coordination and communication
between (vertical) layers such a consistent hold-off timer
configuration (and setup through signalling during the working LSP
establishment) can be considered, allowing the synchronization
between recovery actions performed across these layers.
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7.2.1 Recovery Granularity
In most environments, the design of the network and the vertical
distribution of the LSP bandwidth are such that the recovery
granularity is finer at higher layers. The OTN and SONET/SDH layers
can only recover the whole section or the individual connections it
transports whereas the IP/MPLS control plane can recover individual
packet LSPs or groups of packet LSPs and this independently of their
granularity. On the other side, the recovery granularity at the sub-
wavelength level (i.e. SONET/SDH) can be provided only when the
network includes devices switching at the same granularity (and thus
not with optical channel level). Therefore, the network layer can
deliver control-plane driven recovery mechanisms on a per-LSP basis
if and only if these LSPs have their corresponding switching
granularity supported at the transport plane level.
7.3 Escalation Strategies
There are two types of escalation strategies (see [DEMEESTER]):
bottom-up and top-down.
The bottom-up approach assumes that lower layer recovery types and
schemes are more expedient and faster than the upper layer one.
Therefore we can inhibit or hold-off higher layer recovery. However
this assumption is not entirely true. Consider for instance a
Sonet/SDH based protection mechanism (with a less than 50 ms
protection switching time) lying on top of an OTN restoration
mechanism (with a less than 200 ms restoration time). Therefore,
this assumption should be (at least) clarified as: lower layer
recovery mechanism is expected to be faster than upper level one if
the same type of recovery mechanism is used at each layer.
Consequently, taking into account the recovery actions at the
different layers in a bottom-up approach, if lower layer recovery
mechanisms are provided and sequentially activated in conjunction
with higher layer ones, the lower layers must have an opportunity to
recover normal traffic before the higher layers do. However, if
lower layer recovery is slower than higher layer recovery, the lower
layer must either communicate the failure related information to the
higher layer(s) (and allow it to perform recovery), or use a hold-
off timer in order to temporarily set the higher layer recovery
action in a "standby mode". Note that the a priori information
exchange between layers concerning their efficiency is not within
the current scope of this document. Nevertheless, the coordination
functionality between layers must be configurable and tunable.
An example of coordination between the optical and packet layer
control plane enables for instance the optical layer performing the
failure management operations (in particular, failure detection and
notification) while giving to the packet layer control plane the
authority to decide and perform the recovery actions. In case the
packet layer recovery action is unsuccessful, fallback at the
optical layer can be subsequently performed.
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The top-down approach attempts service recovery at the higher layers
before invoking lower layer recovery. Higher layer recovery is
service selective, and permits "per-CoS" or "per-connection" re-
routing. With this approach, the most important aspect is that the
upper layer should provide its own reliable and independent failure
detection mechanism from the lower layer.
The same reference also suggests recovery mechanisms incorporating a
coordinated effort shared by two adjacent layers with periodic
status updates. Moreover, some of these recovery operations can be
pre-assigned (on a per-link basis) to a certain layer, e.g. a given
link will be recovered at the packet layer while another will be
recovered at the optical layer.
7.4 Disjointness
Having link and node diverse working and recovery LSPs/spans does
not guarantee their complete disjointness. Due to the common
physical layer topology (passive), additional hierarchical concepts
such as the Shared Risk Link Group (SRLG) and mechanisms such as
SRLG diverse path computation must be developed to provide complete
working and recovery LSP/span disjointness (see [IPO-IMP] and
[GMPLS-RTG]). Otherwise, a failure affecting the working LSP/span
would also potentially affect the recovery LSP/span; one refers to
such an event as "common failure".
7.4.1 SRLG Disjointness
A Shared Risk Link Group (SRLG) is defined as the set of links
sharing a common risk (for instance, a common physical resource such
as a fiber link or a fiber cable). For instance, a set of links L
belongs to the same SRLG s, if they are provisioned over the same
fiber link f.
The SRLG properties can be summarized as follows:
1) A link belongs to more than one SRLG if and only if it crosses
one of the resources covered by each of them.
2) Two links belonging to the same SRLG can belong individually to
(one or more) other SRLGs.
3) The SRLG set S of an LSP is defined as the union of the
individual SRLG s of the individual links composing this LSP.
SRLG disjointness is also applicable to LSPs:
The LSP SRLG disjointness concept is based on the following
postulate: an LSP (i.e. sequence of links and nodes) covers an
SRLG if and only if it crosses one of the links or nodes
belonging to that SRLG.
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Therefore, the SRLG disjointness for LSPs can be defined as
follows: two LSPs are disjoint with respect to an SRLG s if and
only if they do not cover simultaneously this SRLG s.
Whilst the SRLG disjointness for LSPs with respect to a set S of
SRLGs is defined as follows: two LSPs are disjoint with respect
to a set of SRLGs S if and only if the common SRLGs between the
sets of SRLGs they individually cover is disjoint from set S.
The impact on recovery is noticeable: SRLG disjointness is a
necessary (but not a sufficient) condition to ensure network
survivability. With respect to the physical network resources, a
working-recovery LSP/span pair must be SRLG disjoint in case of
dedicated recovery type. On the other hand, in case of shared
recovery, a group of working LSP/span must be mutually SRLG-disjoint
in order to allow for a (single and common) shared recovery LSP
itself SRLG-disjoint from each of the working LSPs/spans.
8. Recovery Mechanisms Analysis
In order to provide a structured analysis of the recovery mechanisms
detailed in the previous sections, the following dimensions can be
considered:
1. Fast convergence (performance): provide a mechanism that
aggregates multiple failures (this implies fast failure
detection and correlation mechanisms) and fast recovery decision
independently of the number of failures occurring in the optical
network (implying also a fast failure notification).
2. Efficiency (scalability): minimize the switching time required
for LSP/span recovery independently of the number of LSPs/spans
being recovered (this implies an efficient failure correlation, a
fast failure notification and time-efficient recovery
mechanism(s)).
3. Robustness (availability): minimize the LSP/span downtime
independently of the underlying topology of the transport plane
(this implies a highly responsive recovery mechanism).
4. Resource optimization (optimality): minimize the resource
capacity, including LSPs/spans and nodes (switching capacity),
required for recovery purposes; this dimension can also be
referred to as optimizing the sharing degree of the recovery
resources.
5. Cost optimization: provide a cost-effective recovery type/scheme.
However, these dimensions are either outside the scope of this
document such as cost optimization and recovery path computational
aspects or mutually conflicting. For instance, it is obvious that
providing a 1+1 LSP protection minimizes the LSP downtime (in case
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of failure) while being non-scalable and consuming recovery resource
without enabling any extra-traffic.
The following sections provide an analysis of the recovery phases
and mechanisms detailed in the previous sections with respect to the
dimensions described here above to assess the GMPLS protocol suite
capabilities and applicability. In turn, this allows the evaluation
of the potential need for further GMPLS signaling and routing
extensions.
8.1 Fast Convergence (Detection/Correlation and Hold-off Time)
Fast convergence is related to the failure management operations. It
refers to the elapsing time between the failure detection/
correlation and hold-off time, point at which the recovery switching
actions are initiated. This point has been detailed in Section 4.
8.2 Efficiency (Recovery Switching Time)
In general, the more pre-assignment/pre-planning of the recovery
LSP/span, the more rapid the recovery is. Since protection implies
pre-assignment (and cross-connection) of the protection resources,
in general, protection recover faster than restoration.
Span restoration is likely to be slower than most span protection
types; however this greatly depends on the efficiency of the span
restoration signalling. LSP restoration with pre-signaled and pre-
selected recovery resources is likely to be faster than fully
dynamic LSP restoration, especially because of the elimination of
any potential crankback during the recovery LSP establishment.
If one excludes the crankback issue, the difference between dynamic
and pre-planned restoration depends on the restoration path
computation and selection time. Since computational considerations
are outside the scope of this document, it is up to the vendor to
determine the average and maximum path computation time in different
scenarios and to the operator to decide whether or not dynamic
restoration is advantageous over pre-planned schemes depending on
the network environment. This difference depends also on the
flexibility provided by pre-planned restoration versus dynamic
restoration: the former implies a somewhat limited number of failure
scenarios (that can be due, for instance, to local storage capacity
limitation). The latter enables on-demand path computation based on
the information received through failure notification message and as
such is more robust with respect to the failure scenario scope.
Moreover, LSP segment restoration, in particular, dynamic
restoration (i.e. no path pre-computation so none of the recovery
resource is pre-reserved) will generally be faster than end-to-end
LSP restoration. However, local LSP restoration assumes that each
LSP segment end-point has enough computational capacity to perform
this operation while end-to-end LSP restoration requires only that
LSP end-points provides this path computation capability.
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Recovery time objectives for SONET/SDH protection switching (not
including time to detect failure) are specified in [G.841] at 50 ms,
taking into account constraints on distance, number of connections
involved, and in the case of ring enhanced protection, number of
nodes in the ring. Recovery time objectives for restoration
mechanisms have been proposed through a separate effort [RFC3386].
8.3 Robustness
In general, the less pre-assignment (protection)/pre-planning
(restoration) of the recovery LSP/span, the more robust the recovery
type or scheme is to a variety of single failures, provided that
adequate resources are available. Moreover, the pre-selection of the
recovery resources gives in the case of multiple failure scenarios
less flexibility than no recovery resource pre-selection. For
instance, if failures occur that affect two LSPs sharing a common
link along their restoration paths, then only one of these LSPs can
be recovered. This occurs unless the restoration path of at least
one of these LSPs is re-computed or the local resource assignment is
modified on the fly.
In addition, recovery types and schemes with pre-planned recovery
resources, in particular LSP/spans for protection and LSPs for
restoration purposes, will not be able to recover from failures that
simultaneously affect both the working and recovery LSP/span. Thus,
the recovery resources should ideally be as disjoint as possible
(with respect to link, node and SRLG) from the working ones, so that
any single failure event will not affect both working and recovery
LSP/span. In brief, working and recovery resource must be fully
diverse in order to guarantee that a given failure will not affect
simultaneously the working and the recovery LSP/span. Also, the risk
of simultaneous failure of the working and the recovery LSP can be
reduced. This, by computing a new recovery path whenever a failure
occurs along one of the recovery LSPs or by computing a new recovery
path and provision the corresponding LSP whenever a failure occurs
along a working LSP/span. Both methods enable the network to
maintain the number of available recovery path constant.
The robustness of a recovery scheme is also determined by the amount
of pre-reserved (i.e. signaled) recovery resources within a given
shared resource pool: as the sharing degree of recovery resources
increases, the recovery scheme becomes less robust to multiple
LSP/span failure occurrences. Recovery schemes, in particular
restoration, with pre-signaled resource reservation (with or without
pre-selection) should be capable to reserve the adequate amount of
resource to ensure recovery from any specific set of failure events,
such as any single SRLG failure, any two SRLG failures etc.
8.4 Resource Optimization
It is commonly admitted that sharing recovery resources provides
network resource optimization. Therefore, from a resource
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utilization perspective, protection schemes are often classified
with respect to their degree of sharing recovery resources with
respect to the working entities. Moreover, non-permanent bridging
protection types allow (under normal conditions) for extra-traffic
over the recovery resources.
From this perspective 1) 1+1 LSP/Span protection is the most
resource consuming protection type since not allowing for any extra-
traffic 2) 1:1 LSP/span recovery requires dedicated recovery
LSP/span allowing for extra-traffic 3) 1:N and M:N LSP/span recovery
require 1 (M, respectively) recovery LSP/span (shared between the N
working LSP/span) allowing for extra-traffic. Obviously, 1+1
protection precludes and 1:1 recovery does not allow for any
recovery LSP/span sharing whereas 1:N and M:N recovery do allow
sharing of 1 (M, respectively) recovery LSP/spans between N working
LSP/spans. However, despite the fact that 1:1 LSP recovery precludes
the sharing of the recovery LSP, the recovery schemes (see Section
5.4) that can be built from it (e.g. (1:1)^n) do allow sharing of
its recovery resources. In addition, the flexibility in the usage of
shared recovery resources (in particular, shared links) may be
limited because of network topology restrictions, e.g. fixed ring
topology for traditional enhanced protection schemes.
On the other hand, when using LSP restoration with pre-signaled
resource reservation, the amount of reserved restoration capacity is
determined by the local bandwidth reservation policies. In LSP
restoration schemes with re-provisioning, a pool of spare resources
can be defined from which all resources are selected after failure
occurrence for the purpose of restoration path computation. The
degree to which restoration schemes allow sharing amongst multiple
independent failures is then directly inferred from the size of the
resource pool. Moreover, in all restoration schemes, spare resources
can be used to carry preemptible traffic (thus over preemptible
LSP/span) when the corresponding resources have not been committed
for LSP/span recovery purposes.
From this, it clearly follows that less recovery resources (i.e.
LSP/spans and switching capacity) have to be allocated to a shared
recovery resource pool if a greater sharing degree is allowed. Thus,
the network survivability level is determined by the policy that
defines the amount of shared recovery resources and by the maximum
sharing degree allowed for these recovery resources.
8.4.1. Recovery Resource Sharing
When recovery resources are shared over several LSP/Spans, the use
of the Maximum Reservable Bandwidth, the Unreserved Bandwidth and
the Maximum LSP Bandwidth (see [GMPLS-RTG]) provides the information
needed to obtain the optimization of the network resources allocated
for shared recovery purposes.
The Maximum Reservable Bandwidth is defined as the Maximum Link
Bandwidth but it may be greater in case of link over-subscription.
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The Unreserved Bandwidth (at priority p) is defined as the bandwidth
not yet reserved on a given TE link (its initial value for each
priority p corresponds to the Maximum Reservable Bandwidth). Last,
the Maximum LSP Bandwidth (at priority p) is defined as the smaller
of Unreserved Bandwidth (at priority p) and Maximum Link Bandwidth.
Here, one generally considers a recovery resource sharing degree (or
ratio) to globally optimize the shared recovery resource usage. The
distribution of the bandwidth utilization per TE link can be
inferred from the per-priority bandwidth pre-allocation. By using
the Maximum LSP Bandwidth and the Maximum Reservable Bandwidth, the
amount of (over-provisioned) resources that can be used for shared
recovery purposes is known from the IGP.
In order to analyze this behavior, we define the difference between
the Maximum Reservable Bandwidth (in the present case, this value is
greater than the Maximum Link Bandwidth) and the Maximum LSP
Bandwidth per TE link i as the Maximum Shareable Bandwidth or
max_R[i]. Within this quantity, the amount of bandwidth currently
allocated for shared recovery per TE link i is defined as R[i]. Both
quantities are expressed in terms of discrete bandwidth units (and
thus, the Minimum LSP Bandwidth is of one bandwidth unit).
The knowledge of this information available per TE link can be
exploited in order to optimize the usage of the resources allocated
per TE link for shared recovery. If one refers to r[i] as the actual
bandwidth per TE link i (in terms of discrete bandwidth units)
committed for shared recovery, then the following quantity must be
maximized over the potential TE link candidates:
sum {i=1}^N [(R{i} - r{i})/(t{i} - b{i})]
or equivalently: sum {i=1}^N [(R{i} - r{i})/r{i}]
with R{i} >= 1 and r{i} >= 1 (in terms of per component
bandwidth unit)
In this formula, N is the total number of links traversed by a given
LSP, t[i] the Maximum Link Bandwidth per TE link i and b[i] the sum
per TE link i of the bandwidth committed for working LSPs and other
recovery LSPs (thus except "shared bandwidth" LSPs). The quantity
[(R{i} - r{i})/r{i}] is defined as the Shared (Recovery) Bandwidth
Ratio per TE link i. In addition, TE links for which R[i] reaches
max_R[i] or for which r[i] = 0 are pruned during shared recovery
path computation as well as TE links for which max_R[i] = r[i] which
can simply not be shared.
More generally, one can draw the following mapping between the
available bandwidth at the transport and control plane level:
- ---------- Max Reservable Bandwidth
| ----- ^
|R ----- |
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| ----- |
- ----- |max_R
----- |
-------- TE link Capacity - ------ | - Maximum TE Link Bandwidth
----- |r ----- v
----- <------ b ------> - ---------- Maximum LSP Bandwidth
----- -----
----- -----
----- -----
----- -----
----- ----- <--- Minimum LSP Bandwidth
-------- 0 ---------- 0
Note that the above approach does not require the flooding of any
per LSP information or any detailed distribution of the bandwidth
allocation per component link or individual ports or even any per-
priority shareable recovery bandwidth information (using a dedicated
sub-TLV). The latter would provide the same capability than the
already defined Maximum LSP bandwidth per-priority information. Such
approach is referred to as a Partial (or Aggregated) Information
Routing as described for instance in [KODIALAM1] and [KODIALAM2].
They show that the difference obtained with a Full (or Complete)
Information Routing approach (where for the whole set of working and
recovery LSPs, the amount of bandwidth units they use per-link is
known at each node and for each link) is clearly negligible. The
latter approach is detailed in [GLI], for instance. Note also that
both approaches rely on the deterministic knowledge (at different
degrees) of the network topology and resource usage status.
Moreover, extending the GMPLS signalling capabilities can enhance
the Partial Information Routing approach. This, by allowing working
LSP related information and in particular, its path (including link
and node identifiers) to be exchanged with the recovery LSP request
to enable more efficient admission control at upstream nodes of
shared recovery resources, in particular links (see Section 8.4.3).
8.4.2 Recovery Resource Sharing and SRLG Recovery
Resource shareability can also be maximized with respect to the
number of times each SRLG is protected by a recovery resource (in
particular, a shared TE link) and methods can be considered for
avoiding contention of the shared recovery resources in case of
single SRLG failure. These methods enable for the sharing of
recovery resources between two (or more) recovery LSPs if their
respective working LSPs are mutually disjoint with respect to link,
node and SRLGs. A single failure then does not simultaneously
disrupt several (or at least two) working LSPs.
For instance, [BOUILLET] shows that the Partial Information Routing
approach can be extended to cover recovery resource shareability
with respect to SRLG recoverability (i.e. the number of times each
SRLG is recoverable). By flooding this aggregated information per TE
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link, path computation and selection of SRLG-diverse recovery LSPs
can be optimized with respect to the sharing of recovery resource
reserved on each TE link giving a performance difference of less
than 5% (and so negligible) compared to the corresponding Full
Information Flooding approach (see [GLI]).
For this purpose, additional extensions to [GMPLS-RTG] in support of
path computation for shared mesh recovery have been often considered
in the literature. TE link attributes would include, among other,
the current number of recovery LSPs sharing the recovery resources
reserved on the TE link and the current number of SRLGs recoverable
by this amount of (shared) recovery resources reserved on the TE
link. The latter is equivalent to the current number of SRLGs that
the recovery LSPs sharing the recovery resource reserved on the TE
link shall recover. Then, if explicit SRLG recoverability is
considered an additional TE link attribute including the explicit
list of SRLGs recoverable by the shared recovery resource reserved
on the TE link and their respective shareable recovery bandwidth.
The latter information is equivalent to the shareable recovery
bandwidth per SRLG (or per group of SRLGs) which implies to consider
a decreasing amount of shareable bandwidth and SRLG list over time.
Compared to the case of recovery resource sharing only (regardless
of SRLG recoverability, as described in Section 8.4.1), this
additional TE link attributes would potentially deliver better path
computation and selection (at distinct ingress node) for shared mesh
recovery purposes. However, due to the lack of results of evidence
for better efficiency and due to the complexity that such extensions
would generate, they are not further considered in the scope of the
present analysis. For instance, a per-SRLG group minimum/maximum
shareable recovery bandwidth is restricted by the length that the
corresponding (sub-)TLV may take and thus the number of SRLGs that
it can include. Therefore, the corresponding parameter should not be
translated into GMPLS routing (or even signalling) protocol
extensions in the form of TE link sub-TLV.
8.4.3 Recovery Resource Sharing, SRLG Disjointness and Admission
Control
Admission control is a strict requirement to be fulfilled by nodes
giving access to shared links. This can be illustrated using the
following network topology:
A ------ C ====== D
| | |
| | |
| B |
| | |
| | |
------- E ------ F
Node A creates a working LSP to D (A-C-D), B creates simultaneously
a working LSP to D (B-C-D) and a recovery LSP (B-E-F-D) to the same
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destination. Then, A decides to create a recovery LSP to D (A-E-F-
D), but since the C-D span carries both working LSPs, node E should
either assign a dedicated resource for this recovery LSP or reject
this request if the C-D span has already reached its maximum
recovery bandwidth sharing ratio. Otherwise, in the latter case, C-D
span failure would imply that one of the working LSP would not be
recoverable.
Consequently, node E must have the required information to perform
admission control for the recovery LSP requests it processes
(implying for instance, that the path followed by the working LSP is
carried with the corresponding recovery LSP request). If node E can
guarantee that the working LSPs (A-C-D and B-C-D) are SRLG disjoint
over the C-D span, it may securely accept the incoming recovery LSP
request and assign to the recovery LSPs (A-E-F-D and B-E-F-D) the
same resources on the link E-F. This, if the link E-F has not yet
reached its maximum recovery bandwidth sharing ratio. In this
example, one assumes that the node failure probability is negligible
compared to the link failure probability.
To achieve this, the path followed by the working LSP is transported
with the recovery LSP request and examined at each upstream node of
potentially shareable links. Admission control is performed using
the interface identifiers (included in the path) to retrieve in the
TE DataBase the list of SRLG Ids associated to each of the working
LSP links. If the working LSPs (A-C-D and B-C-D) have one or more
link or SRLG id in common (in this example, one or more SRLG id in
common over the span C-D) node E should not assign the same resource
over link E-F to the recovery LSPs (A-E-F-D and B-E-F-D). Otherwise,
one of these working LSPs would not be recoverable in case of C-D
span failure.
There are some issues related to this method, the major one being
the number of SRLG Ids that a single link can cover (more than 100,
in complex environments). Moreover, when using link bundles, this
approach may generate the rejection of some recovery LSP requests.
This occurs when the SRLG sub-TLV corresponding to a link bundle
includes the union of the SRLG id list of all the component links
belonging to this bundle (see [GMPLS-RTG] and [BUNDLE]).
In order to overcome this specific issue, an additional mechanism
may consist of querying the nodes where such an information would be
available (in this case, node E would query C). The main drawback of
this method is that, in addition to the dedicated mechanism(s) it
requires, it may become complex when several common nodes are
traversed by the working LSPs. Therefore, when using link bundles,
solving this issue is tightly related to the sequence of the
recovery operations. Per component flooding of SRLG identifiers
would deeply impact the scalability of the link state routing
protocol. Therefore, one may rely on the usage of an on-line
accessible network management system.
D.Papadimitriou et al. - Expires October 2005 36
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt April 2005
9. Summary and Conclusions
The following table summarizes the different recovery types and
schemes analyzed throughout this document.
--------------------------------------------------------------------
| Path Search (computation and selection)
--------------------------------------------------------------------
| Pre-planned (a) | Dynamic (b)
--------------------------------------------------------------------
| | faster recovery | Does not apply
| | less flexible |
| 1 | less robust |
| | most resource consuming |
Path | | |
Setup ------------------------------------------------------------
| | relatively fast recovery | Does not apply
| | relatively flexible |
| 2 | relatively robust |
| | resource consumption |
| | depends on sharing degree |
------------------------------------------------------------
| | relatively fast recovery | less faster (computation)
| | more flexible | most flexible
| 3 | relatively robust | most robust
| | less resource consuming | least resource consuming
| | depends on sharing degree |
--------------------------------------------------------------------
1a. Recovery LSP setup (before failure occurrence) with resource
reservation (i.e. signalling) and selection is referred to as
LSP protection.
2a. Recovery LSP setup (before failure occurrence) with resource
reservation (i.e. signalling) and with resource pre-selection is
referred to as pre-planned LSP re-routing with resource pre-
selection. This implies only recovery LSP activation after
failure occurrence.
3a. Recovery LSP setup (before failure occurrence) with resource
reservation (i.e. signalling) and without resource selection is
referred to as pre-planned LSP re-routing without resource pre-
selection. This implies recovery LSP activation and resource
(i.e. label) selection after failure occurrence.
3b. Recovery LSP setup after failure occurrence is referred to as
to as LSP re-routing, which is full when recovery LSP path
computation occurs after failure occurrence.
The term pre-planned refers thus to recovery LSP path pre-
computation, signaling (reservation), and a priori resource
selection (optional), but not cross-connection. Also, the shared-
D.Papadimitriou et al. - Expires October 2005 37
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt April 2005
mesh recovery scheme can be viewed as a particular case of 2a) and
3a) using the additional constraint described in Section 8.4.3.
The implementation of these recovery mechanisms requires only
considering extensions to GMPLS signalling protocols (i.e. [RFC3471]
and [RFC3473]). These GMPLS signalling extensions should mainly
focus in delivering (1) recovery LSP pre-provisioning for the cases
1a, 2a and 3a, (2) LSP failure notification, (3) recovery LSP
switching action(s), and (4) reversion mechanisms.
Moreover, the present analysis (see Section 8) shows that no GMPLS
routing extensions are expected to efficiently implement any of
these recovery types and schemes.
10. Security Considerations
This document does not introduce any additional security issue or
imply any specific security consideration from [RFC3945] to the
current RSVP-TE GMPLS signaling, routing protocols (OSPF-TE, IS-IS-
TE) or network management protocols.
However, the authorization of requests for resources by GMPLS-
capable nodes should determining whether a given party, presumable
already authenticated, has a right to access the requested
resources. This determination is typically a matter of local policy
control, for example by setting limits on the total bandwidth made
available to some party in the presence of resource contention. Such
policies may become quite complex as the number of users, types of
resources and sophistication of authorization rules increases. This
is particularly the case for recovery schemes that assume pre-
planned sharing of recovery resources, or contention for resources
in case of dynamic re-routing.
Therefore, control elements should match them against the local
authorization policy. These control elements must be capable of
making decisions based on the identity of the requester, as verified
cryptographically and/or topologically.
11. IANA Considerations
This document defines no new code points and requires no action by
IANA.
12. Acknowledgments
The authors would like to thank Fabrice Poppe (Alcatel) and Bart
Rousseau (Alcatel) for their revision effort, Richard Rabbat
(Fujitsu Labs), David Griffith (NIST) and Lyndon Ong (Ciena) for
their useful comments.
Thanks also to Adrian Farrel for the thorough review of the
document.
D.Papadimitriou et al. - Expires October 2005 38
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13. References
13.1 Normative References
[BUNDLE] K.Kompella et al., "Link Bundling in MPLS Traffic
Engineering," Work in progress, draft-ietf-mpls-bundle-
06.txt, December 2004.
[GMPLS-RTG] K.Kompella (Editor) et al., "Routing Extensions in
Support of Generalized Multi-Protocol Label Switching,"
Work in Progress, draft-ietf-ccamp-gmpls-routing-
09.txt, October 2003.
[LMP] J.P.Lang (Editor) et al., "Link Management Protocol
(LMP)," Work in progress, draft-ietf-ccamp-lmp-10.txt,
October 2003.
[LMP-WDM] A.Fredette and J.P.Lang (Editors), "Link Management
Protocol (LMP) for Dense Wavelength Division
Multiplexing (DWDM) Optical Line Systems," Work in
progress, draft-ietf-ccamp-lmp-wdm-03.txt, October
2003.
[RFC2026] S.Bradner, "The Internet Standards Process -- Revision
3," BCP 9, RFC 2026, October 1996.
[RFC2119] S.Bradner, "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
[RFC3471] L.Berger (Editor) et al., "Generalized Multi-Protocol
Label Switching (GMPLS) Signaling Functional
Description," RFC 3471, January 2003.
[RFC3473] L.Berger (Editor) et al., "Generalized Multi-Protocol
Label Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions," RFC
3473, January 2003.
[RFC3667] S.Bradner, "IETF Rights in Contributions", BCP 78,
RFC 3667, February 2004.
[RFC3668] S.Bradner, Ed., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3668, February 2004.
[RFC3945] E.Mannie (Editor) et al., "Generalized Multi-Protocol
Label Switching Architecture," RFC 3945, October 2004.
[TERM] E.Mannie and D.Papadimitriou (Editors), "Recovery
(Protection and Restoration) Terminology for
Generalized Multi-Protocol Label Switching (GMPLS),"
Work in progress, draft-ietf-ccamp-gmpls-recovery-
terminology-06.txt, April 2005.
D.Papadimitriou et al. - Expires October 2005 39
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt April 2005
13.2 Informative References
[BOUILLET] E.Bouillet et al., "Stochastic Approaches to Compute
Shared Meshed Restored Lightpaths in Optical Network
Architectures," IEEE Infocom 2002, New York City, June
2002.
[DEMEESTER] P.Demeester et al., "Resilience in Multilayer
Networks," IEEE Communications Magazine, Vol. 37, No.
8, pp. 70-76, August 1998.
[GLI] G.Li et al., "Efficient Distributed Path Selection for
Shared Restoration Connections," IEEE Infocom 2002, New
York City, June 2002.
[IPO-IMP] J.Strand and A.Chiu, "Impairments and Other Constraints
On Optical Layer Routing," Work in Progress, draft-
ietf-ipo-impairments-05.txt, May 2003.
[KODIALAM1] M.Kodialam and T.V.Lakshman, "Restorable Dynamic
Quality of Service Routing," IEEE Communications
Magazine, pp. 72-81, June 2002.
[KODIALAM2] M.Kodialam and T.V.Lakshman, "Dynamic Routing of
Restorable Bandwidth-Guaranteed Tunnels using
Aggregated Network Resource Usage Information," IEEE/
ACM Transactions on Networking, pp. 399-410, June 2003.
[MANCHESTER] J.Manchester, P.Bonenfant and C.Newton, "The Evolution
of Transport Network Survivability," IEEE
Communications Magazine, August 1999.
[RFC3386] W.Lai, D.McDysan, J.Boyle, et al., "Network Hierarchy
and Multi-layer Survivability," RFC 3386, November 2002
[RFC3469] V.Sharma and F.Hellstrand (Editors), "Framework for
Multi-Protocol Label Switching (MPLS)- based Recovery,"
RFC 3469, February 2003.
[T1.105] ANSI, "Synchronous Optical Network (SONET): Basic
Description Including Multiplex Structure, Rates, and
Formats," ANSI T1.105, January 2001.
[WANG] J.Wang, L.Sahasrabuddhe, and B.Mukherjee, "Path vs.
Subpath vs. Link Restoration for Fault Management in
IP-over-WDM Networks: Performance Comparisons Using
GMPLS Control Signaling," IEEE Communications Magazine,
pp. 80-87, November 2002.
For information on the availability of the following documents,
please see http://www.itu.int
D.Papadimitriou et al. - Expires October 2005 40
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt April 2005
[G.707] ITU-T, "Network Node Interface for the Synchronous
Digital Hierarchy (SDH)," Recommendation G.707, October
2000.
[G.709] ITU-T, "Network Node Interface for the Optical
Transport Network (OTN)," Recommendation G.709,
February 2001 (and Amendment no.1, October 2001).
[G.783] ITU-T, "Characteristics of Synchronous Digital
Hierarchy (SDH) Equipment Functional Blocks,"
Recommendation G.783, October 2000.
[G.806] ITU-T, "Characteristics of Transport Equipment -
Description Methodology and Generic Functionality",
Recommendation G.806, October 2000.
[G.808.1] ITU-T, "Generic Protection Switching - Linear trail and
Subnetwork Protection," Recommendation G.808.1,
December 2003.
[G.841] ITU-T, "Types and Characteristics of SDH Network
Protection Architectures," Recommendation G.841,
October 1998.
[G.842] ITU-T, "Interworking of SDH network protection
architectures," Recommendation G.842, October 1998.
14. Editor's Addresses
Eric Mannie
EMail: eric_mannie@hotmail.com
Dimitri Papadimitriou
Alcatel
Francis Wellesplein, 1
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
EMail: dimitri.papadimitriou@alcatel.be
D.Papadimitriou et al. - Expires October 2005 41
draft-ietf-ccamp-gmpls-recovery-analysis-05.txt April 2005
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D.Papadimitriou et al. - Expires October 2005 42